U.S. patent number 6,977,792 [Application Number 10/618,439] was granted by the patent office on 2005-12-20 for method and apparatus for runout correction by proper positioning of servo data while self-servo writing.
This patent grant is currently assigned to Maxtor Corporation. Invention is credited to James W. Hargarten, Bruce A. Liikanen, Thomas O. Melrose.
United States Patent |
6,977,792 |
Melrose , et al. |
December 20, 2005 |
Method and apparatus for runout correction by proper positioning of
servo data while self-servo writing
Abstract
A method and system for self-writing track locations of a data
disk in a disk drive in order to reduce overall track runout are
disclosed. First set of servo bursts are self-written along a track
via a transducer, and repeatable runout correction values for the
first servo bursts are calculated. Then, second servo bursts are
self-written along the track via the transducer such that the first
and second servo bursts form a plurality of servo sector patterns
that define the track centerline, wherein the second servo bursts
are positioned using said correction values to essentially
compensate for the runout in the first servo bursts and reduce the
overall track runout.
Inventors: |
Melrose; Thomas O. (Longmont,
CO), Hargarten; James W. (Lafayette, CO), Liikanen; Bruce
A. (Berthoud, CO) |
Assignee: |
Maxtor Corporation (Longmont,
CO)
|
Family
ID: |
35465615 |
Appl.
No.: |
10/618,439 |
Filed: |
July 10, 2003 |
Current U.S.
Class: |
360/75;
360/77.04; G9B/5.221 |
Current CPC
Class: |
G11B
5/59627 (20130101); G11B 5/59672 (20130101) |
Current International
Class: |
G11B
005/596 () |
Field of
Search: |
;360/75,77.02,77.04,77.08 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
Hargarten et al., "Method and Apparatus for Determining Embedded
Runout Correction Values Using Feedback," U.S Appl. No. 10/318,316,
filed Dec. 11, 2002. .
Ehrlich et al., "Method and Apparatus for Providing Variable Gain
Iterative Embedded Runout Correction in a Disk Drive," U.S. Appl.
No. 10/410,576, filed Apr. 8, 2003..
|
Primary Examiner: Hudspeth; David
Assistant Examiner: Habermehl; James L
Parent Case Text
RELATED APPLICATION
Priority is claimed from U.S. Provisional Application No.
60/394,850, entitled "SSW Burst Position Correction", filed on Jul.
10, 2002, which is incorporated herein by reference.
Claims
What is claimed is:
1. A method for self writing track locations of a storage surface
of a data disk of a disk drive, comprising the steps of: (a)
self-writing first servo bursts along a circular track via a
transducer; (b) calculating repeatable runout correction values for
the first servo bursts; (c) self-writing second servo bursts along
the track via the transducer such that the first and second servo
bursts form a plurality of servo sector patterns that define the
track centerline, wherein the second servo bursts are positioned
using said correction values to essentially compensate for the
runout in the first servo bursts and reduce the overall track
runout.
2. The method of claim 1, wherein in step (c) the second servo
bursts are written using the correction values such that the
average track centerline is adjusted to essentially compensate for
the runout in the first servo bursts.
3. The method of claim 1, wherein each servo sector pattern
includes a trimmed burst pattern.
4. The method of claim 3, wherein: the step of writing the first
servo bursts in each servo sector pattern further includes the
steps of writing two servo bursts wherein one of the servo bursts
trims the other servo burst, defining a first seam; the step of
calculating the repeatable runout correction values further
includes the steps of calculating repeatable runout correction
values based on differences between the intended positions of the
first seams and the actual positions of the first seams as written,
wherein said difference represent mis-placements of the first
seams; and the step of writing the second servo bursts in each
servo sector pattern further includes the steps of writing two
servo bursts wherein one of the servo bursts trims the other servo
burst, defining a second seam, such that the second seams are
positioned using said correction values to essentially compensate
for mis-placement of the first seams, thereby reducing the overall
track runout.
5. The method of claim 4, wherein each servo sector pattern
includes a trimmed burst pattern comprising four radially offset,
circumferentially staggered, servo bursts.
6. The method of claim 1, wherein: the step of writing the first
servo bursts includes the additional steps of determining a
position error due to repeatable runout at each of a plurality of
points in the first servo burst pattern along the track addressed
by the transducer head, and storing the position error; and the
step of calculating the repeatable runout correction values for the
first servo burst pattern further includes the steps of: measuring
a time domain impulse response of a disk drive servo control system
associated with said transducer head; transforming said time domain
impulse response into an error transfer function; taking the
reciprocal of said error transfer function; transforming said
reciprocal error transfer function into an inverse impulse
response; and convolving said inverse impulse response with said
position error at each of a plurality of said points in the first
servo pattern to obtain a runout correction value for each of said
plurality of points.
7. The method of claim 6, wherein the data disk includes a
reference pattern for determining said position error for
self-writing the servo patterns, the method further including the
steps of generating the position error values based on the
reference pattern.
8. The method of claim 7, wherein the step of generating the
position error values based on the reference pattern further
includes the steps of reducing any existing repeatable runout in
the position error information that is obtained from the reference
pattern to obtain enhanced position information, and using the
enhanced position information as said position error values.
9. The method of claim 6, wherein said step of transforming said
time domain impulse response into an error transfer function is
performed using a Discrete Fourier Transform.
10. The method of claim 6, wherein said step of transforming said
reciprocal error transfer function into an inverse impulse is
performed using an inverse Discrete Fourier Transform.
11. A method for self writing track locations of a storage surface
of a data disk of a disk drive, wherein the data disk includes a
reference pattern for determining position information error for
self-writing, the method comprising the steps of: generating
position information based on the reference pattern; reducing any
existing repeatable runout in the position information that is
obtained from the reference pattern to obtain enhanced position
information, and using the enhanced position information for
self-writing servo bursts; self-writing first servo bursts along a
circular track via a transducer and concurrently determining a
position error due to repeatable runout at each of a plurality of
points in the first servo burst pattern along the track addressed
by the transducer head, and storing the position error; and
calculating the repeatable runout correction values for the first
servo burst pattern by: measuring a time domain impulse response of
a disk drive servo control system associated with said transducer
head; transforming said time domain impulse response into an error
transfer function; taking the reciprocal of said error transfer
function; transforming said reciprocal error transfer function into
an inverse impulse response; and convolving said inverse impulse
response with said position error at each of a plurality of said
points in the first servo pattern to obtain a runout correction
value for each of said plurality of points; and self-writing second
servo bursts along the track via the transducer such that the first
and second servo bursts form a plurality of servo sector patterns
that define the track centerline, wherein the second servo bursts
are positioned using said correction values to essentially
compensate for the runout in the first servo bursts and reduce the
overall track runout.
12. The method of claim 11, wherein the second servo bursts are
written using the correction values such that the average track
centerline is adjusted to essentially compensate for the runout in
the first servo bursts.
13. The method of claim 11, wherein each servo sector pattern
includes a trimmed burst pattern.
14. The method of claim 13, wherein: the step of writing the first
servo bursts in each servo sector pattern further includes the
steps of writing two servo bursts wherein one of the servo bursts
trims the other servo burst, defining a first seam; the step of
calculating the repeatable runout correction values further
includes the steps of calculating repeatable runout correction
values based on differences between the intended positions of the
first seams and the actual positions of the first seams as written,
wherein said difference represent misplacements of the first seams;
and the step of writing the second servo bursts in each servo
sector pattern further includes the steps of writing two servo
bursts wherein one of the servo bursts trims the other servo burst,
defining a second seam, such that the second seams are positioned
using said correction values to essentially compensate for
mis-placement of the first seams, thereby reducing the overall
track runout.
15. The method of claim 14, wherein each servo sector pattern
includes a trimmed burst pattern comprising four radially offset,
circumferentially staggered, servo bursts.
16. The method of claim 11, wherein said step of transforming said
time domain impulse response into an error transfer function is
performed using a Discrete Fourier Transform.
17. The method of claim 11, wherein said step of transforming said
reciprocal error transfer function into an inverse impulse is
performed using an inverse Discrete Fourier Transform.
18. A hard disk drive having servo burst position correction,
comprising: a base; a data disk comprising a reference pattern for
providing position information to self-write final servo patterns
in a plurality of data tracks arranged concentrically about a
spindle, wherein each of said data tracks is segmented into a
plurality of data sectors by servo sectors, wherein said disks may
be rotated at a constant velocity with respect to said base; a
transducer head for reading information from said data disk and for
writing information to said data disk, wherein said transducer head
is movable in a radial direction with respect to said disk to
address a selected one of said plurality of data tracks; a voice
coil motor, interconnected to said transducer head, for moving said
transducer head with respect to said data tracks; a channel for
receiving signals, including position error signals, derived from
said disk by said transducer head; and a controller, interconnected
to said voice coil motor, for controlling a position of said
transducer head with respect to said reference pattern, wherein the
controller writes final servo bursts on the data disk by
self-writing first servo bursts along a circular track via the
transducer, calculating repeatable runout correction values for the
first servo bursts and self-writing second servo bursts along the
track via the transducer such that the first and second servo
bursts form a plurality of servo sector patterns that define the
track centerline, wherein the second servo bursts are positioned
using said correction values to essentially compensate for the
runout in the first servo bursts and reduce the overall track
runout.
19. The disk drive of claim 18, wherein the second servo bursts are
written using the correction values such that the average track
centerline is adjusted to essentially compensate for the runout in
the first servo bursts.
20. The disk drive of claim 18, wherein each servo sector pattern
includes a trimmed burst pattern.
21. The disk drive of claim 20, wherein: the first servo bursts
include two servo bursts wherein the controller causes the
transducer to write one of the servo bursts to trim the other servo
burst, defining a first seam; the controller calculates the
repeatable runout correction values based on differences between
the intended positions of the first seams and the actual positions
of the first seams as written, wherein said difference represent
misplacements of the first seams and; the second servo bursts
include two servo bursts wherein the controller causes the
transducer to write one of the servo bursts to trim the other servo
burst, defining a second seam, such that the second seams are
positioned using said correction values to essentially compensate
for mis-placement of the first seams, thereby reducing the overall
track runout.
22. The disk drive of claim 21, wherein each servo sector pattern
includes a trimmed burst pattern comprising four radially offset,
circumferentially staggered, servo bursts.
23. The disk drive of claim 18, wherein: while writing the first
servo bursts, the controller determines a position error due to
repeatable runout at each of a plurality of points in the first
servo burst pattern along the track addressed by the transducer
head, and stores the position error; and the controller calculates
the repeatable runout correction values for the first servo burst
pattern by convolving an inverse impulse response of a disk drive
servo system with said position error at each of a plurality of
said points in the first servo pattern to obtain a runout
correction value for each of said plurality of points, wherein the
inverse impulse response is obtained by measuring a time domain
impulse response of a disk drive servo control system associated
with said transducer head, transforming said time domain impulse
response into an error transfer function, taking the reciprocal of
said error transfer function, and transforming said reciprocal
error transfer function into an inverse impulse response.
24. The disk drive of claim 23, wherein transforming said time
domain impulse response into an error transfer function is
performed using a Discrete Fourier Transform.
25. The disk drive of claim 23, wherein said step of transforming
said reciprocal error transfer function into an inverse impulse is
performed using an inverse Discrete Fourier Transform.
Description
FIELD OF THE INVENTION
The present invention relates to self-servo writing disk drives and
more particularly to enhancement of runout correction by proper
positioning of servo data while self-servo writing disk drives.
BACKGROUND OF THE INVENTION
Background for the present invention is provided herein in
connection with a disk drive system. It should be noted, however,
that the present invention is not intended to be limited to such
systems.
A disk drive is a data storage device that stores digital data in
tracks on the surface of a data storage disk. Data is read from or
written to a track of the disk using a transducer that is held
close to the track while the disk spins about its center at a
substantially constant angular velocity. To properly locate the
transducer near the desired track during a read or write operation,
a closed-loop servo scheme is generally implemented that uses servo
data read from the disk surface to align the transducer with the
desired track.
The servo data includes servo patterns that typically comprise
short servo bursts of a constant frequency signal, which are very
precisely located and offset from either side of a data track's
centerline. The bursts are written in a sector header area, and can
be used to find the centerline of a track. Staying on center is
required during both reading and writing. These servo-data areas
allow a head to follow a track centerline around a disk, even when
the track is out-of-round, as can occur with spindle wobble, disk
slip and/or thermal expansion.
Servo bursts are conventionally written on a disk in the disk drive
by a dedicated, external servo track writer (STW), which typically
involves the use of large granite blocks to support the disk drive
and to quiet outside vibration effects. However, servo track
writers are expensive and require a clean room environment. As
such, self-servo writing (SSW) methods for writing servo patterns
with a disk drive's own transducers have been utilized.
Typically, in a SSW process, a temporary set of pre-existing
reference servo information on a disk is used to control the
transducer position while the final servo bursts are written to
disk(s) in the disk drive. The SSW process involves a combination
of three largely distinct sub-processes, including reading the
temporary servo information to provide precise timing information,
positioning a transducer at a sequence of radial positions using
the variation in a read back signal amplitude as a sensitive
position indicator, and writing the final servo burst patterns at
the times and radial positions defined by the other two processes
to form concentric circular tracks. An example SSW process is
described in U.S. Pat. No. 5,907,447, by Yarmchuk, et al. Other SSW
to processes are possible, such as servo propagation where the
servo reader to writer offset is used to allow servoing on one set
of servo bursts while writing another set of servo bursts.
In an ideal disk drive system, the tracks of the data disk are
non-perturbed circles situated about the center of the disk. As
such, each of these ideal tracks includes a track centerline that
is located at a known constant radius from the disk center. In an
actual system, however, it is difficult to write non-perturbed
circular tracks to the data storage disk. That is, problems, such
as vibration, bearing defects, etc. can result in tracks that are
written differently from the ideal non-perturbed circular track
shape. Positioning errors created by the perturbed nature of these
tracks are known as written-in repetitive runout (SSW.sub.-- RRO).
The perturbed shape of these tracks complicates the transducer
positioning function during read and write operations after the SSW
process because the servo system needs to continuously reposition
the transducer during track following to keep up with the
constantly changing radius of the track centerline with respect to
the center of the spinning disk. Furthermore, the perturbed shape
of the these tracks can result in problems such as track squeeze
and track misregistration errors during read and write
operations.
In certain systems, as will be understood by those skilled in the
art, after the servo patterns are written, an additional process is
used to directly measure the SSW.sub.-- RRO for each track of a
disk so that compensation values are generated and written in servo
fields on the disk. Thereafter, during read/write operations, that
compensation information is used to position the transducer along
an ideal track centerline. An example of such a process is
described in U.S. Pat. No. 6,549,362 to Melrose et al. ('362
patent), which is incorporated herein by reference.
However, such a correction technique though effective, can be time
consuming. First, the amount of SSW.sub.-- RRO present on each
track of a disk must be measured, and then a calculation is
performed to determine correction factors to minimize the
SSW.sub.-- RRO in each track. Finally, the correction factors must
be written to the disk in each servo field of each track. This
process requires several revolutions to measure the SSW.sub.-- RRO
and then more revolutions to write the correction factors to the
disk. In one example, such a process may require 12 or more
revolutions to determine and write correction factors for each
track.
There is, therefore, a need for a method and apparatus which
improves embedded runout correction in a disk drive during the
self-servo writing process and which also reduces the correction
time required.
BRIEF SUMMARY OF THE INVENTION
The present invention addresses the above needs. In one embodiment,
the present invention provides a method and system for reducing
overall track runout while self writing track locations of a
storage surface of a data disk of a disk drive. First, a set of
servo bursts are self-written along a circular track via a
transducer and repeatable runout error values for the first servo
bursts are measured essentially instantaneously while the servo
bursts are being written. A set of correction values are calculated
for use in servoing, such that the transducer is caused to travel
along a path that, on average, is equal but opposite to the path
measured while the first set of servo bursts were being
written.
Then, using the calculated set of correction values for positioning
the transducer, a second set of servo bursts is self-written along
the track via the transducer, such that the first and second servo
bursts form a plurality of servo sector patterns that define the
track centerline. The second servo bursts are positioned using said
correction values to essentially compensate for the runout in the
first servo bursts and to reduce the overall track runout. The
second servo bursts are written using the correction values such
that the average track centerline is adjusted to essentially
compensate for the runout in the first servo bursts. Preferably,
the correction values are obtained by circularly convolving the
measured runout present during the writing of the first set of
servo bursts with the inverse impulse response of the servo
system.
In another example of the present invention, each servo sector
pattern includes a trimmed burst pattern. As such, the step of
writing the first servo bursts in each servo sector pattern further
includes the steps of writing two servo bursts wherein one of the
servo bursts trims the other servo burst, defining a first seam.
Further, the step of calculating the repeatable runout correction
values further includes the steps of measuring the repeatable
runout (RRO) values based on differences between the intended
positions of the first seams and the actual position of the
transducer while the first seams are written, wherein said
differences represent misplacements of the first seams. Then,
correction values are obtained by convolving said measured RRO of
the first seams with the inverse impulse response of the servo
system to generate correction values for positioning the transducer
while writing the second set of servo bursts (i.e., second seams).
Then, the step of writing the second servo bursts in each servo
sector pattern further includes the steps of writing two servo
bursts wherein one of the servo bursts trims the other servo burst,
defining a second seam, such that the second seams are positioned
using said correction values to essentially compensate for
misplacement of the first seams, thereby reducing the overall track
runout.
According to another example of the present invention, the steps of
writing the first servo bursts includes the additional steps of
determining a position error due to repeatable runout at each of a
plurality of points in the first servo burst pattern along the
track addressed by the transducer head, and storing the position
error. Further, the step of calculating the repeatable runout
correction values for the first servo burst pattern further
includes the steps of measuring a time domain impulse response of a
disk drive servo control system associated with said transducer
head, transforming said time domain impulse response into an error
transfer function, taking the reciprocal of said error transfer
function, transforming said reciprocal error transfer function into
an inverse impulse response, and circularly convolving said inverse
impulse response with said position error at each of a plurality of
said points in the first servo pattern to obtain a runout
correction value for each of said plurality of points. This process
circularizes the track such that Non Repeatable Runout (NRO) error
that perturbs the path of the head while the first set of servo
bursts are being written is the only source of Repeatable Runout
(RRO). Therefore, any PES (Position Error Signal) that exists when
the track is being written is identical to the RRO being
written-in.
In another aspect, the present invention provides a disk drive
including a controller which implements the self-servo writing
method of the present invention.
Other objects, features and advantages of the invention will be
apparent from the following specification taken in conjunction with
the following drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a diagrammatic representation of a top view of a hard
disk drive, with the cover removed;
FIG. 2 is a diagrammatic representation of a magnetic storage disk
having a self-servo written data track in accordance with the
present invention;
FIG. 3 is a diagrammatic representation of a servo burst pattern
that may be used to position a transducer head with respect to a
track centerline;
FIG. 4 is a block diagram depicting the relationship between
repetitive runout and the position error signal for a particular
track;
FIG. 5 is a block diagram depicting the relationship between the
position error signal for a particular track and the repetitive
runout;
FIG. 6 is a flow chart illustrating a method for determining
embedded runout correction values in accordance with an embodiment
of the present invention;
FIGS. 7A and 7B comprise a diagrammatic representation of a servo
burst pattern written by an example self-servo writing process
according to the present invention;
FIG. 8 is a flowchart of the steps of another example self-servo
writing (SSW) process according to another embodiment of the
present invention; and
FIGS. 9A and 9B comprise a diagrammatic representation of a servo
burst pattern written according to the self-servo writing steps in
FIG. 8.
DETAILED DESCRIPTION OF THE INVENTION
While this invention is susceptible of embodiments in many
different forms, there are shown in the drawings and will herein be
described in detail, preferred embodiments of the invention with
the understanding that the present disclosure is to be considered
as an exemplification of the principles of the invention and is not
intended to limit the broad aspects of the invention to the
embodiments illustrated.
Further, although in the description below, example embodiments of
the present invention are described in connection with a disk drive
system, it should be noted, however, that the present invention is
not intended to be limited to such system.
FIG. 1 illustrates a typical computer disk drive. The disk drive,
generally identified by reference number 100, includes a base 104
and magnetic disks 108 (only one of which is shown in FIG. 1). The
magnetic disks 108 are interconnected to the base 104 by a spindle
motor (not shown) mounted within or beneath the hub 112, such that
the disks 108 can be rotated relative to the base 104. Actuator arm
assemblies 116 (only one of which is shown in FIG. 1) are
interconnected to the base 104 by a bearing 120. The actuator arm
assemblies 116 each include a transducer head 124 at a first end,
to address each of the surfaces of the magnetic disks 108. A voice
coil motor (VCM) 128 pivots the actuator arm assemblies 116 about
the bearing 120 to radially position the transducer heads 124 with
respect to the magnetic disks 108. By changing the radial position
of the transducer heads 124 with respect to the magnetic disks 108,
the transducer heads 124 can access different data tracks or
cylinders 132 on the magnetic disks 108. The voice coil motor 128
is operated by a controller 136 that is in turn operatively
connected to a host computer (not shown). A channel 140 processes
information read from the magnetic disks 108 by the transducer
heads 124.
As illustrated in FIG. 2, the disk 108 is substantially circular in
shape and includes a center point 200 located in the center of the
disk 108. The disk 108 also includes a plurality of tracks 132
(only one of which is illustrated in FIG. 2) on an upper surface
204 of the disk 108 for storing the digital data. The data tracks
132 are divided into data fields 208a-208d and servo sectors or
hard sectors 212a-212d. Generally, the data fields 208a-208d are
used for storing data as a series of magnetic transitions, while
the servo sectors 212a-212d are used for storing servo information,
also as a series of magnetic transitions/bursts, that is used to
provide the transducer head 124 with positioning information. In
particular, the servo sectors 212a-212d provide the transducer
heads 124 with information concerning their position over the
magnetic disk 108. More particularly, the servo sectors 212a-212d
provide information to the transducer heads 124 concerning the
identity of the track 132 and servo sector 212 over which each
transducer head 124 is flying, and concerning the position of each
transducer head with respect to the centerline of the track
132.
Although the magnetic disk 108 illustrated in FIG. 2 is illustrated
as having a single data track 132 and four servo sectors 212, it
can be appreciated that a typical computer disk drive contains a
very large number of data tracks 132 and servo sectors 212. For
example, computer disk drives having over 100,000 tracks per inch
and 240 sectors are presently available.
Referring to both FIGS. 1 and 2, the disk drive 100 includes a
servo control system 144 for controlling the position of a
transducer head 124 with respect to a track 132 being followed. In
general, the servo control system comprises the transducer head 124
being positioned, which reads the position information from the
servo sectors 212; the actuator arm assembly 116 that is carrying
the transducer head 124; the voice coil motor 128; the channel 140;
and the controller 136. As described in the '362 patent, the
response of the servo control system 144 to a given input is given
by the error transfer function of the servo control system 144.
The track 132 is ideally non-perturbed and ideally shares a common
center 200 with the disk 108, such as ideal track 216 illustrated
in FIG. 2. Due to system imperfections, however, the actual written
track 132 can be perturbed as compared to an ideal track 216 such
as non-ideal track 132 as illustrated in FIG. 2. A perturbed or
non-ideal track 132 is difficult for a transducer head 124 to
follow, because the position of the transducer head 124 must
constantly be adjusted by the servo control system. Consequently,
the positioning of the transducer head 124 is not as accurate on
the written track 132 as it would be on an ideal track 216.
The perturbations in the written track 132 due to positioning
errors can be reduced by an enhanced servo writing process
according to the present invention. In one embodiment, the present
invention provides a method and system that allows self-writing of
servo information (e.g., servo bursts) in tracks 132 that more
closely resemble the ideal track 216 by reducing said position
errors (SSW.sub.-- RRO). As such, thereafter a transducer head 124
servoing on a track 132 can more closely follow the path of an
ideal track, such as the path of track 216, using the self-written
servo information.
As mentioned above, the tracks 132 on the disk 108 are each divided
into a plurality of data fields 208 and servo sectors or hard
sectors 212. The servo sectors 212 include, among other things,
information for use by the disk drive 100 in locating a transducer
head 124 above a desired track 132 of the disk 108. When a host
computer requests that data be read from or written to a particular
track 132 and data field 208 of the disk 108, the transducer head
124 must be moved to the track 132 and then must be positioned at a
predetermined location relative to the centerline of the track 132
before data transfer can take place. For purposes of illustrating
the present invention, it will be assumed that the transducer
should be placed on the track centerline in order to read from and
write to the disk. It should be understood that the invention is
not limited to solely reading and writing when the transducer is
placed at the track centerline. As noted above, the track 132 is
written to the disk 108 in a SSW process according to the present
invention, such that SSW.sub.-- RRO is reduced.
FIG. 3 illustrates typical servo hard sectors 130a stored within
the servo portion of a servo sector 212 for use in centering a
transducer head 124 on a desired track 132. In this example, a
servo hard sector 130a includes sets 130b of staggered servo bursts
130c, which are designated as bursts A, B, C and D. The servo
bursts 130c define the centerlines Tn-1, Tn and Tn+1 of the tracks
132 of the disk 108. In FIG. 3, the tracks 132 are diagrammatically
laid out linearly in a down-track direction from left to night, and
in a cross-track direction from top to bottom, of the page. Three
example centerlines Tn-1, Tn and Tn+1 of three tracks 132 are
defined by the servo bursts 130c on each track. The servo bursts
130c provide analog information to the servo system for
transducer/head positioning. A different number of servo bursts and
offset configurations are also possible. In the example herein, the
A, B bursts form a burst pair and the C, D bursts form another
burst pair. During normal disk drive operations, all of the four
bursts A, B, C and D are used by the servo system when the
transducer 124 is positioned at a write track centerline.
An example SSW process, according to the present invention for
writing a servo track 132 that more closely resembles the ideal
track 216, is now generally described. A temporary reference
pattern of servo information (not shown) is initially provided on
the disk 108 and is used by the servo system to determine a
position error signal (PES) for positioning the transducer 124 to
write the servo bursts. Further, the example SSW process is
described in conjunction with a "trimmed" servo burst system. As
used herein, a trimmed servo burst is one in which a radial edge of
the burst is DC erased during a subsequent pass of the write
element at a displaced radial position relative to the disk. A
burst is trimmed to have e.g. a lower radial edge to be in
alignment with the upper radial edge of an adjacent burst. It is
possible to trim a previously written burst during a single pass of
the transducer write head along a servo-writing path for writing
another burst. A discussion of trimmed and untrimmed bursts is
provided in U.S. Pat. No. 6,519,107 to Ehrlich, et al.,
incorporated herein by reference.
The method for self writing servo bursts includes the steps of
self-writing first servo bursts along a track via a transducer,
calculating repeatable runout correction values for the first servo
bursts, and self-writing second servo bursts along the track via
the transducer such that the first and second servo bursts form a
plurality of servo sector patterns that define the track
centerline, wherein the second servo bursts are positioned using
said correction values to essentially compensate for the runout in
the first servo bursts and reduce the overall track runout.
As such, in one example, in self-writing a track 132, the
transducer 124 is positioned to write the bursts A ("A bursts")
along a circular path during a revolution of the disk 108. Then, in
another revolution the transducer 124 is moved a portion of the
track width (e.g. 2/3 of track width) to write the bursts B ("B
bursts"), wherein the B bursts trim off bottom edges of the A
bursts, thereby defining a first burst seam (transition) 130d. The
positioning errors in the seams 130d, which are measured while the
servo system is self-writing the B bursts, are the offset values
which are then stored in memory. Then, in another revolution, the
bursts C ("C bursts") are written. Finally, in another revolution,
the stored offset values are used to calculate correction values in
writing the bursts D ("D bursts"). The D bursts are positioned to
trim the C bursts and generate seams 130e that compensate for the
positioning errors in the seams 130d.
The observed position of the seam 130d is controlled by the
position of the transducer 124 during the trimming operation. As
such, if burst B is being trimmed by burst A, then the position of
the seam 130d is controlled by the position of the transducer 124
while writing burst A. Conversely, if burst A is trimmed by burst B
then the position of the seam 130d is controlled by the position of
the transducer 124 while writing burst B. The same holds for the
bursts C and D.
Other sequences for writing and trimming the bursts 130c are
possible, as described in greater detail further below. As such, in
another example, the bursts D are written first in a disk
revolution, then the bursts B are written in another disk
revolution, then the bursts C are written in another disk
revolution such that the bursts C trim the bursts D creating the
seams 130e, and then in another disk revolution the bursts A are
written such that the bursts A trim the bursts B, creating the
seams 130d. The motion of the head 14 defines where the seam 130e
between the bursts C and D occurs. As there is head motion due to
disturbances that cause non-repeatable run out disturbances (NRO),
the difference between the intended position of the seam 130e and
the actual position of the seam 130e due to such head movement, is
a capture of the NRO, and is recorded in the burst pair pattern C,
D by mis-positioning of the seam 130e (SSW.sub.-- RRO). According
to an embodiment of the present invention, to correct that
SSW.sub.-- RRO in the seam 130e, the transducer 124 is positioned
such that bursts A trim bursts B wherein the seams 130d for the A,
B bursts are positioned (laid down) to compensate for the
mis-positioning of the seam 130e. For example, if the seam 130e is
too far off towards the outer diameter (OD) of the disk 108, then
the seam 130d are positioned further towards the inner diameter
(ID) of the disk 108 by an offset value to compensate for the
mis-positioning of the seam 130e (SSW.sub.-- RRO).
After the final servo bursts are written by the SSW, in normal disk
drive operations, the servo system senses the position of the seams
130d between the bursts A and B, and the seams 130e between the
bursts C and D, for track following. At each read/write position,
one seam 130d and one seam 130e is used, wherein the servo system
averages the observed position of the seams 130d, 130e to generate
a position error signal (PES) to control the VCM 128 for properly
positioning the transducer 124 over the tracks 132. Therefore, if
during the SSW process, a seam 130e were mis-positioned slightly
towards the ID of its intended (ideal) position, then according to
the present invention, the seam 130d is intentionally
mis-positioned (offset from ideal) by an equal amount toward the
OD. As a result, the transducer head 124 is made to follow the path
of an ideal track 216 by averaging the PES signal due to the seams
130d, 130e in each of the servo sectors 212 of a particular track
132.
To determine the non-repeatable run out (SSW.sub.-- RRO) values of
the seams 130e, in one implementation, the SSW.sub.-- RRO in the
seams 130e is determined based on the instantaneous PES at the time
the seams 130e are created (e.g., when writing the C bursts that
trim the D bursts). That instantaneous PES is determined using said
pre-existing temporary servo information used for head positioning
during the SSW process. The PES is used to compensate for the
SSW.sub.-- RRO while laying out the seams 130d, as described
further below.
In one example, the instantaneous PES corresponding to the seams
130e, as detected during a disk revolution, is circularly convolved
with the inverse impulse response of the disk drive servo system to
determine the correction offset for adjusting the centerline of the
132 track when laying down the seams 130d, such that the position
of the seams 130d cancel out the SSW.sub.-- RRO written into the
seams 130e. Because the PES of the track is due to the RMS sum of
the two seams (e.g., 130e and 130d), and because the effect of NRO
recorded in the seams 130e is effectively reduced by the
positioning of the seams 130e, the method of the present invention
reduces the total track SSW.sub.-- RRO (e.g., by about 25%).
An example implementation of the above SSW process is now described
in more detail. In a preferred implementation of the SSW process,
an iterative process such as described in the '362 patent is
applied to the temporary servo information that is used for
servoing while writing the final servo bursts A, B, C and D. This
reduces the RRO that may have been written into the temporary servo
information itself. The temporary servo information, with reduced
RRO, is then used for writing the servo bursts. However, as
mentioned, in writing the servo bursts that define the seams 130e,
NRO is recorded as SSW.sub.-- RRO as mis-positioned seams 130e.
Without compensation, the mis-positioned seams 130e perturb the
track 132. The instantaneous PES from the temporary servo
information when laying down the seams 130e indicates how far the
seams 130e are mis-positioned from their ideal/intended position.
The instantaneous PES (PES.sub.-- RRO) is obtained from the
temporary servo information, the instant the C bursts trim the D
bursts to lay down the seams 130e, and are stored in memory.
Of importance is where the burst B is trimmed by the burst A, and
not necessarily the position of the burst B. As such, the
instantaneous PES (PES.sub.-- RRO), while performing a burst
write/trim operation that controls a seam position, indicates how
far the first seams 130e were mis-positioned. The mis-positioning
error (SSW.sub.-- RRO) is calculated from the position error values
(PES.sub.-- RRO) and compensated for by intentionally
mis-positioning the seams 130d on the other burst pair, in an equal
and opposite manner relative to the ideal position of the first
seams 130d.
To calculate the SSW.sub.-- RRO values from the measured position
error values (PES.sub.-- RRO), the impulse response of the servo
system is used. Then, correction/offset values are calculated to
compensate for mis-positioning of the first seams (e.g., seams
130e). The correction values are used to create a control signal
for the VCM 128 to position the transducer 124 when laying down the
second seams (e.g., seams 130d) to compensate for the
mis-positioning of the first seams (e.g., seams 130e). Therefore,
in the above example, when writing the bursts A that trim the
bursts B, the average track centerline is displaced by the
correction/offset values to compensate for the NRO that was
recorded (SSW.sub.-- RRO) when the bursts C trimmed the bursts D,
thereby reducing the overall track runout.
The instantaneous PES values due to repeatable runout in the first
seams 130e (i.e., PES.sub.-- RRO values) are related to the
SSW.sub.-- RRO values by a predetermined transfer function S(z)
400, as illustrated in FIG. 4. The transfer function 400, in
general, describes how the servo control system 144 reacts to and
follows the track 132. That is, SSW.sub.-- RRO is the stimulus and
PES.sub.-- RRO is the response. As illustrated in FIG. 5, in order
to determine the SSW.sub.-- RRO values using the measured
PES.sub.-- RRO values, one needs to find the inverse transfer
function S.sup.-1 (z) 500 and to apply the PES.sub.-- RRO values
thereto.
An example process of obtaining the impulse response and the
inverse impulse response is in the '362 patent, and as such is only
briefly described herein. The inverse impulse can be determined by
measuring (in the time domain) the response of the servo control
system associated with a transducer head to an impulse signal or
function written to a track on the disk or otherwise provided to
the servo control system. The error transfer function may then be
obtained by transforming the measured impulse response to the
frequency domain. The transformation of the impulse response of the
transducer head is performed using a Discrete Fourier Transform.
The reciprocal of the resulting error transfer function may then be
taken. The reciprocal error transfer function response may then be
transformed back into the time domain to obtain the inverse impulse
response of the servo system for the transducer head. The
transformation to the time domain is accomplished by using an
inverse Discrete Fourier Transform. The runout correction values
for the first seams are then calculated by circularly convolving
the corresponding position error (PES.sub.-- RRO) due to repeatable
runout with the inverse impulse response for the transducer head.
An inverse impulse response of a transducer head can be obtained by
introducing an impulse to the transducer head from a track located
towards an outer diameter of the disk surface addressed by the
transducer head. An inverse impulse response can also be determined
for each transducer head in the hard disk drive.
FIG. 6 illustrates the steps of correcting SSW.sub.-- RRO in a
self-servo writing process in accordance with an embodiment of the
present invention. At step 600, the instantaneous position error
signal due to repeatable runout in the first seams (PES.sub.-- RRO)
is determined. In general, the position error due to repeatable
runout (PES.sub.-- RRO) is derived by reading the temporary servo
information when the first seams (e.g., seams 130e) are laid down
for a track 132, and then generating a position error signal
therefrom. The process of obtaining position error signal/data from
the temporary servo information is known by those skilled in the
art, and as such not described herein. Preferably, the position
error data is obtained from the temporary servo information,
wherein the RRO that may have been recorded in the temporary servo
information is reduced by known methods.
At step 604, the inverse impulse response for the servo system is
determined. An example method for determining the inverse impulse
response of the system is discussed in the '362 patent. Once the
inverse impulse response for the system is determined, a circular
convolution of the PES.sub.-- RRO and the inverse impulse response
for the system is performed at step 608. The result of the
convolution operation are the SSW.sub.-- RRO values (see FIG. 5).
The SSW.sub.-- RRO values obtained for the first seams 130e are
used to calculate correction/offset values that are then utilized
in controlling the VCM 128 in positioning the transducer 124 when
laying down the second seams (e.g., seams 130d). As described, this
process compensates for the mis-positioning of the first seams
(e.g., seams 130e) and reduces the track runout (step 612).
FIGS. 7A and 7B comprise a diagrammatic representation of an
example self-servo writing (SSW) process according to the present
invention. Four tracks 132, designated as tracks N, N+1, N+2 and
N+3, are shown. Tracks N+1 and N+3 are shown with no SSW position
error (SSW.sub.-- RRO), and tracks N and N+2 are shown with SSW
positioning error (in opposite radial directions). The
`inconsequential bursts` serve to isolate adjacent tracks and
prevent any correction accumulation.
In writing the servo bursts for track N using track mode TM7,
first, all the A bursts are written in a revolution. Then, in
another revolution all the C bursts for the track N are written.
Then, in another revolution the B bursts are written wherein each B
burst trims the bottom edge of a corresponding A burst (represented
as a dashed box, designated "A trimmed"). As each A burst is
trimmed, the instantaneous PES (PES.sub.-- RRO) at that location is
stored in memory, wherein the instantaneous PES information
indicates the position of the A, B seam 130d. The recorded
PES.sub.-- RRO values are then used to determine correction/offset
values that are used in another revolution for writing the D bursts
at positions that trim the C bursts, such that the position of C, D
seams 130e compensate for mis-positioning of the corresponding A, B
seams 130d. The D bursts are written wherein each D burst trims the
edge of a corresponding C burst (represented as a dashed box,
designated "C trimmed"), creating the C, D seams 130e.
FIG. 8 shows a more detailed flowchart of the steps of another
example self-servo writing (SSW) process, according to the present
invention. FIGS. 9A and 9B comprise a diagrammatic example of a
method of self-writing servo burst patterns using steps such as
those shown in FIG. 8. For simplicity, the steps in FIG. 8 are also
shown in FIGS. 9A and 9B, from bottom to top in sequence, and each
step is aligned with the respective burst writing/trimming
operation. The SSW process is described for four tracks 132,
designated as tracks N, N+1, N+2 and N+3 in FIGS. 9A and 9B. Tracks
N+1 and N+3 are shown with no SSW position error (SSW.sub.-- RRO),
and tracks N and N+2 are shown with SSW positioning error (in
opposite radial directions). The `inconsequential bursts` serve to
isolate adjacent tracks and prevent any correction
accumulation.
In FIGS. 9A and 9B, the servo bursts A, B, C and D define the
tracks N, N+1, N+2 and N+3 as shown, and correspond to different
track "modes" (e.g., TM1, TM3, TM5 and TM7). Each track mode
indicates the sequence in which the bursts are written/trimmed, and
the corresponding PES is based on combinations of the burst
difference values corresponding to the track mode. For example,
track mode TM1 corresponds to the burst combination
PES=-(A-B)+(C-D), the track mode TM3 corresponds to the burst
combination PES=(A-B)+(C-D), the track mode TM5 corresponds to the
burst combination PES=(A-B)-(C-D), and the track mode TM7
corresponds to the burst combination PES=-(A-B)-(C-D). Other track
modes TM0, TM2, TM4, TM6 are used for two burst tracks (i.e., A and
B bursts).
In this example, writing the 4-burst pattern herein is performed in
eight steps which represent the four different track modes. The
process starts at a track mode (e.g., TM1), and cycles through the
track modes depending on the steps in the eight-step process, as
shown by the example in FIGS. 9A and 9B and described hereinbelow.
As those skilled in the art will appreciate, the present invention
is applicable to other burst patterns and other burst numbers
(e.g., 6 burst system) by taking into account the burst
relationships and how the bursts trim one another. The present
invention is also applicable to other servo writing methods such as
servo writing by multiple trims of one or more servo bursts.
As an overview, referring to FIGS. 9A and 9B, in writing the servo
bursts for track N using track mode TM7, first all the A bursts are
written in a revolution. Then, in another revolution all the C
bursts for the track N are written. Then, in another revolution the
B bursts are written wherein each B burst trims the bottom edge of
a corresponding A burst (represented as a dashed box, designated "A
trimmed"). As each A burst is trimmed, the instantaneous PES
(PES.sub.-- RRO) at that location is stored in memory, wherein the
instantaneous PES information indicates the position of the A, B
seam 130d. The recorded PES.sub.-- RRO values are then used to
determine correction/offset values that are used in another
revolution for writing the D bursts at positions that trim the C
bursts, such that the position of C, D seams 130e compensate for
mis-positioning of the corresponding A, B seams 130d. The D bursts
are written wherein each D burst trims the edge of a corresponding
C burst (represented as a dashed box, designated "C trimmed"),
creating the C, D seams 130e.
Now referring to the steps in FIG. 8 in conjunction with the
diagram in FIGS. 9A and 9B (starting from track N+3 at the bottom
of FIG. 9B, and moving from the bottom to the top of FIG. 9A), the
detailed steps for writing the servo bursts for the four tracks N,
N+1, N+2 and N+3 are described, wherein:
1) The B bursts are written in a disk revolution (step 700);
2) Then D bursts are written in another disk revolution (step
702);
3) Then A bursts are written in another disk revolution, wherein
each A burst trims a corresponding B bursts (inconsequential burst)
(step 704);
4) Then C bursts are written in another disk revolution, wherein
each C burst trims a corresponding D burst (creating C, D seams
130e), such that the instantaneous PES (PES.sub.-- RRO) is stored
in memory while writing each C burst (step 706); then, the just
stored PES values (PES.sub.-- RRO) are circularly convolved with
the inverse impulse response (IIR) of the system to determine the
corresponding C, D seam positions (offsets) (step 708);
5) Then B bursts are written in another disk revolution such that
each B burst trims a corresponding A burst (creating A, B seams
130d), wherein when writing each B burst the corresponding C, D
seam position (offset) calculated in step 708 is subtracted from
the PES that controls the transducer position writing that B burst
to compensate for SSW.sub.-- RRO in C, D seam position (step
710);
6) Then D bursts are written in another disk revolution such that
each D burst trims a C burst (inconsequential burst) (step
712);
7) Then A bursts are written in another disk revolution such that
each A burst trims a B burst (creating A, B seams 130d), wherein
the instantaneous PES (PES.sub.-- RRO) is stored in memory while
writing each A burst (step 714); then, the just stored PES values
(PES.sub.-- RRO) are circularly convolved with the inverse impulse
response (IIR) of the system to determine the corresponding A, B
seam positions (offsets) (step 716);
8) Then C bursts are written in another disk revolution such that
each C burst trims a corresponding D burst (creating C, D seams
130e), wherein when writing each C burst, the corresponding A, B
seam position (offset) calculated in step 716 is subtracted from
the PES that controls the transducer position writing that C burst,
to compensate for SSW.sub.-- RRO in A, B seam position (step
718);
9) Then B bursts are written in another disk revolution such that
each B burst trims an A burst (inconsequential burst) (step
720);
10) Then D bursts are written in another disk revolution such that
each D burst trims a corresponding C burst (creating C, D seams
130e), wherein the instantaneous PES (PES.sub.-- RRO) is stored in
memory while writing each D burst (step 722); then, the just stored
PES values (PES.sub.-- RRO) are circularly convolved with the
inverse impulse response (IIR) of the system to determine the
corresponding C, D seam positions (offsets) (step 724);
11) Then A bursts are written in another disk revolution such that
each A burst trims a corresponding B burst (creating A, B seams
130d), wherein when writing each A burst, the corresponding C, D
seam position (offset) calculated in step 724 is subtracted from
the PES that controls the transducer position writing that A burst,
to compensate for SSW.sub.-- RRO in C, D seam position (step
726);
12) Then C bursts are written in another disk revolution such that
each C burst trims a D burst (inconsequential burst) (step
728);
13) Then B bursts are written in another disk revolution such that
each B burst trims a corresponding A burst (creating A, B seams
130d), wherein the instantaneous PES (PES.sub.-- RRO) is stored in
memory while writing each B burst (step 730); then, the just stored
PES values (PES.sub.-- RRO) are circularly convolved with the
inverse impulse response (IIR) of the system to determine the
corresponding A, B seam positions (offsets) (step 732);
14) Then D bursts are written in another disk revolution such that
each D burst trims a corresponding C burst (creating C, D seams
130e), wherein when writing each D burst, the corresponding A, B
seam position (offset) calculated in step 732 is subtracted from
the PES that controls the transducer position writing that D burst,
to compensate for SSW.sub.-- RRO in A, B seam position (step 734);
and so on.
As those skilled in the art will appreciate, implicit in the above
steps is that the transducer 124 is moved under PES and timing
control to write the various bursts at different radial locations.
In one example, the head 124 is wide enough such that it can be
positioned to "see" at least a portion of two or more bursts at the
same time.
After the SSW process, during normal operation of the disk drive
100, the transducer head 124 reads the servo bursts in each servo
sector 212 of a desired track 132. If the head 124 is placed at the
seam 130d between bursts A and B, the head readback signal includes
half the signal value of burst A and half the signal value of burst
B. If the head 124 is shifted off towards burst A, magnitude of
burst A increases and magnitude of burst B decreases. The same
applies to burst pair C, D. The A, B and C, D bursts are shifted in
position from each other by fractions of track width, such as e.g.
1/3 of track width in this example. For head positioning, in one
example, the signal value from the flux transitions in the servo
bursts induced to the transducer are used in a decoding process by
demodulating the induced transducer signals to form difference
values (difference signals) including A-B, and C-D phases.
Transducer position tracking information is decoded by using
combinations of the A-B burst phase and the C-D burst phase
depending on the radial (cross track) location of the transducer
relative to track centerline. As such, the difference signals can
be used in combination to obtain said position error signal (PES)
for transducer positioning by the servo system.
The PES indicates the distance between the center of the transducer
head 124 and the centerline (e.g. centerline 320b) of the desired
track. For a requested read/write operation, the PES signal is used
by the disk drive 100 to change the position of the transducer head
124 to one that is closer to the desired (centered) position. This
centering process is repeated for each successive sector on the
track until the requested read/write operation has been performed
in the appropriate data field 208 of the disk 108. It should be
appreciated that other schemes for storing servo information on the
magnetic media, such as schemes having said A, B position bursts;
using zones; constant linear density (CLD) recording, split data
fields; and/or hybrid servo, can also be used in accordance with
the present invention.
The present invention can be applied to any self-servo writing
system where some temporary servo information is used for timing
and transducer positioning to write final servo data patterns. This
may include printed media, partial write systems and
self-propagation servo write systems, and can be applied to systems
using multi pass writes and trims, as those skilled in the art can
appreciate. Because the temporary servo information will inherently
have a certain amount of RRO in it, it is necessary to remove the
RRO with some type of real-time Runout Correction system, in order
to circularize the temporary servo information before using it to
write the final servo bursts. The correction being used to cancel
out the NRO is added to whatever correction is needed to
circularize the temporary servo information.
As known to those skilled in the art, in addition to the logic
blocks shown in the drawings, the various methods and architectures
described herein can be implemented as: computer instructions for
execution by a microprocessor, ASIC units, firmware, and logic
circuits, etc.
The present invention has been described in considerable detail
with reference to certain preferred versions thereof; however,
other versions are possible. Therefore, the spirit and scope of the
appended claims should not be limited to the description of the
preferred versions contained herein.
* * * * *